Kentucky Agate: State Rock and Mineral Treasure of the Commonwealth

Chapter 3. Agate Formation

19 3 Agate Formation Agates are rocks composed of fine to microcrystalline quartz. Quartz is a mineral, and the type of quartz found in agates is chalcedony. In agates the microcrystalline chalcedonic quartz tends to be concentrically banded and to contain microscopic fibers and inclusions of various iron oxides that give it its stained or layered, colored appearance. Agates are found in all parts of the world and are formed as a part of the earth’s geologic processes. Most agates form in igneous rocks, and Kentucky agate is one of the rare types of agate that have distinctly formed in sedimentary rocks. Other types of agate that may have formed in sedimentary rocks include Fairburn agates, in the southern Black Hills of South Dakota and Nebraska, and agates from Montana. Agates are formed when sediments are transformed into rocks during a process called diagenesis. Diagenesis occurs when multiple layers of sediments are deposited on top of one another until sediment buildup creates pressure and temperature changes on underlying sediments. The pressure or compression on deeper rocks many feet below the sediment-water interface forces minerals and water to segregate, some soluble minerals to dissolve, and other minerals to nucleate or concentrate around each other to form silica gels. These ky agate.indd 19 7/27/13 4:47 PM 20 Kentucky Agate silica gels are themselves formed by complex nucleation or selforganization processes, but other geochemical factors such as the oxidation and valence state of iron contribute to the coloration in agate formation. Water and other fluids, such as iron-rich hydrothermal fluids, the mixing of surface and subsurface fluids, and water acidity, salinity, and temperature in the sediment-water interface are also important parts of agate formation. Many technical papers, reports, and books have been written about the various concepts and theories regarding agate formation. The theories explored in these works include: sedimentary geode and agate formation (Greb, 2012); the origin of agates (Zenz, 2011); silica minerals, chalcedony, and moganite polymorphs (Heaney, 1993, 1995); self-organizational processes in chalcedony and agate formation (Wang and Merino, 1990); hydrothermal fluids (Gotze and others, 2009); and the origin of geodal minerals in Kentucky (Fisher, 1977). Some additional geologic research has been conducted on the important mineral and sedimentary features in the agate-yielding area, which could be related to and contribute to the formation of agates in Kentucky. Anderson (1991) discussed hydrothermal minerals in south-central Kentucky, and Dever (1978, 1999) noted quartz nodule or agate-bearing zones. Siliceous concretions in the underlying Nada Member of the Borden Formation were described by Rice (1972). Yellowish to reddish brown, iron-rich, siliceous, and calcific nodules in the Borden Formation near Alcorn were noted by Haney and Rice (1978), and Simmons (1967) described the ironstone and limonitic ky agate.indd 20 7/27/13 4:47 PM Agate Formation 21 concretions in the Borden Formation near Clay City, Kentucky. These iron-rich nodules and concretions demonstrate the presence of iron-rich fluids in the area. Dolomite is an evaporite mineral, forming in hot, arid environments; moganite, a silica polymorph, was detected in an X-ray diffraction analysis of Kentucky agate and has also been found in evaporite-forming environments (Heaney, 1995). Other minerals such as pyrite, sphalerite, and galena have also been identified in the Renfro Member and suggest a hydrothermal fluid component to the agateformation fluids. The dolomitization of the Renfro Member released abundant iron into solutions that may have mixed with hydrothermal fluids to form agates. Any detailed explanation of agate formation is beyond the scope of this book, but some preliminary petrographic and analytic research does suggest that chalcedony and silica gels formed around a nucleus of dolomite crystals or other evaporite minerals in an iron-rich fluid environment. Additional works are listed in the bibliography that may be of use to the reader interested in the further examination of the formation of agates. Select Photographs The intent in reproducing these photographs is to show the beauty and diversity of these rocks and to provide important information about many of the common features that most agates contain. These selected agates were chosen to be captioned because they are spectacular in color and have some unique features, as the captions describe. All ky agate.indd 21 7/27/13 4:47 PM 22 Kentucky Agate agates have both a commonality and diversity in the intensity of color banding that...